Folic acid and the corresponding (5,6,7,8)-tetrahydrofolates are very important in several therapeutic areas. Folic acid facilitates protection against the risk of several congenital malformations, including neural tube defects, such as spina bifida, anencephaly and encephalocele.
One of the therapeutically widely used tetrahydrofolate is (6S)—N5-methyl-(5,6,7,8)-tetrahydrofolic acid calcium salt (1) (C20H25N7O6*Ca; CAS Registry Number: 151533-22-1).

N5-Methyl-tetrahydrofolic acid and salts thereof are therapeutically relevant for the treatment of megaloblastic anaemia, as antidote for increasing the compatibility of folic acid antagonists, particularly of aminopterin and methotrexate in cancer therapy (“antifolate rescue”), for increasing the therapeutic effect of fluorinated pyrimidines and for the treatment of autoimmune diseases such as psoriasis and rheumatoid arthritis, for increasing the compatibility of certain antiparasitic formulations, for instance trimethoprim-sulfamethoxazole, and for reducing the toxicity of dideazatetrahydrofolates in chemotherapy.
N5-Methyl-tetrahydrofolate alkaline earth metal salts are used in particular as food additive or as a vitamin preparation, for the prevention of neural tube defects, for the treatment of depressive illnesses, and for decreasing the homocysteine level.
N5-Methyl-tetrahydrofolic acid and salts thereof are extremely unstable, and in particular are highly susceptible to oxidation [see for instance A. L. Fitzhugh, Pteridines 4 (4), 187-191 (1993)].
It is therefore difficult to produce them at a level of purity which is acceptable for a pharmaceutical active ingredient or a food additive.
Typical degradation pathways of N5-methyltetrahydrofolate, leading to the characteristic impurities 4α-hydroxy-5-methyltetrahydrofolic acid (D1) and pyrazino-s-triazine derivative (D2a and D2b), are outlined in Schemes 1 and 2.


Various methods, such as excluding oxygen as completely as possible or the addition of antioxidants such as ascorbic acid or reduced L-glutathione, have been employed in order to overcome the instability of 5-methyl-tetrahydrofolic acid.
However, it is scarcely possible to completely exclude oxygen during use, and even then this is only possible at very considerable cost, and the addition of antioxidants is likewise not always possible.
Therefore there is the need to find a method for the stabilization of (6S)—N5-methyl-tetrahydrofolate salts.
The use of stabilizing agents was described in the prior art.
For example WO 2011/018482 (Bayer Schering Pharma AG) discloses stabilised particles comprising a crystalline form of an alkaline earth metal salt of (6S)—N5-methyl-tetrahydrofolic acid and at least one protective agent.
The preparation of formulations of N5-methyl-tetrahydrofolate salts is considered problematic as it can result in the partial decomposition of the active ingredient as reported for example in WO 2008/003432 (Bayer Schering Pharma AG).
EP 1 044 975 (Eprova AG) discloses a preparation of stable crystalline forms of (6S)—N5-methyltetrahydrofolic acid calcium salt by treatment of the product at high temperature, which is technically problematic for large scale production.
U.S. Pat. No. 7,947,662 (Gnosis SpA) discloses pharmaceutical compositions of folates derivatives stabilized as glucosamine salts.
U.S. Pat. No. 5,455,236 (Eprova AG) discloses the use of cyclodextrin to include and stabilize tetrahydrofolate derivatives.
The use of these additives in pharmaceutical formulations is not always desirable.